The invention relates to a multi-voltage electrical system for a vehicle.
It is known that in a vehicle electrical system, in which the supply voltage needed for the electrical consumers is generated by means of a generator, the generator should furnish more than one supply voltage, so as to assure reliable component-specific electrical supply. A vehicle electrical system in which a plurality of different supply voltages are furnished is known from Published, Nonexamined German Patent Application DE-OS 195 19 298. In one such known vehicle electrical system, the electrical energy is generated with the aid of a three-phase generator. The resultant voltage is converted into two different direct voltages with the aid of two rectifier bridges, which can be connected to different points of the phase winding of the generator. In some known versions, thyristors are used as the rectifiers. In addition to the rectification, field effect transistors are employed, with the aid of which a load current limitation is made. The field effect transistors are to that end employed in electrical terms downstream of the rectifier bridges. They are connected to the various voltage pickups, at which the different electrical system voltages can be picked up.
The multi-voltage electrical system according to the invention for a vehicle, has the advantage that from the phase voltages of the generator, at least two different direct voltages can be generated, and at the same time the voltage levels are prevented from adversely affecting one another.
These advantages are attained in that two rectifier arrangements are connected to the phase windings of the generator, and one of the rectifier arrangements is constructed with the aid of controllable rectifier elements, in particular using six MOS field effect transistors, of which two at a time are connected in series with one another and are each located between one phase winding and the common terminal B+. The other rectifier arrangement, at which the higher voltage can be picked up, corresponds to the conventional arrangement of plus and minus diodes between the phase windings of the generator and the terminal B2+.
Further advantages of the invention are attained by other provisions. For instance, it is advantageous to perform the generator regulation in such a way that a voltage of 12 or 14 V is established at one voltage pickup, while 36 or 42 V is available at the other voltage pickup. Advantageously, the rectifier elements of the rectifier arrangement that is located between the generator and B+ are also embodied as MOS field effect transistors. The triggering of the field effect transistors is advantageously accomplished by the voltage regulator or by a control and regulating unit, which is also responsible for the regulation of the exciter current.
The invention can especially advantageously be employed in conjunction with an electrical machine that can function both as a motor (starter) and as a generator. In that case, depending on the operating state, the MOS field effect transistors can function as controllable rectifiers or as controllable inverters; in the mode of operation as a motor, the electrical machine can be operated as an asynchronous or a synchronous motor and can be supplied by a battery, via the field effect transistors operating as controllable inverters.
In a further advantageous feature, additional rectifier arrangements are employed, which furnish still another direct voltage. The rectifier elements connected to the terminal B1+ can also be constructed in such a way that one diode per phase is used; the anode is connected to the applicable phase winding, and the cathodes are jointly connected to a field effect transistor connected in series with them and acting as a controllable actuator. If one field effect transistor per phase winding is used, then a wiring can be provided such that the anode of the parasitic diode is connected to the applicable phase winding, and the cathodes of the parasitic diode are connected jointly to a field effect transistor connected in series with them, which again functions as a controllable actuator.
In the regulation of the generator, the power switches can advantageously be triggered in such a way that the predetermined voltage values at the two terminals B1+ and B2+ can each be established independently of the respective network load and the generator rpm.